1. Field of the Invention
Embodiments of this invention relate generally to erase bands on magnetic media of the type generally used for storing digital data, and in particular embodiments to methods for measuring the width of erase bands on magnetic media, and systems incorporating the same.
2. Description of Related Art
Disk drives typically contain at least one magnetic disk that rotates relative to one or more read/write head assemblies. There is typically one read/write head assembly for each disk surface. The read/write head assemblies magnetize areas on the disk during writing and sense the magnetization of areas on the disk during reading. Conventional read/write head assemblies include a coil wrapped about a C-shaped core, the ends of the core forming two magnetic poles separated by a gap. When writing information onto a disk, current flow in the coil causes magnetic flux to flow in the core and fringe across the gap. Part of the magnetic flux flows into a portion of the disk located under the gap and magnetizes that portion of the disk in a direction parallel to the magnetic flux lines. If the direction of current flow is subsequently reversed, the direction of magnetic flux is also reversed, and areas of the disk will be magnetized in the opposite direction.
When a conventional read/write head assembly is configured to read information from the disk, the magnetized portions of the disk passing under the gap induce a voltage across the coil. The polarity of the induced voltage is dependent on the orientation of the magnetized portion of the disk. As magnetized portions of the disk pass under the gap, the induced voltages are sensed and converted into data.
Information is stored within tracks on the disk, where each track has a width that is approximately equivalent to the width of the read/write head. Typically, such tracks define a pattern of concentric ring-shaped or spiral areas of disk recording surfaces. However, when writing data onto a particular track location, read/write heads will magnetize an area of the disk wider than the width of the head due to magnetic field effects at the edges of the read/write head (edge effects). Data on adjacent track portions may therefore be inadvertently corrupted due to these edge effects. Adjacent track portions can also generate noise and false data transitions when the read/write head is reading information from a particular track location. These effects may be intensified if the head is not properly aligned over the center of the track.
The writing and reading problems associated with adjacent track portions can be minimized if erase bands are formed between tracks. Erase bands are areas of randomized magnetic moments located on either side of a track, and are not intended to carry any recorded information. Erase bands are typically created simultaneously with writing data onto a disk. During writing, in addition to writing data onto the desired track, the read/write head erases any information on the sides of the track by causing the magnetic moments in the bands adjacent to the track to be randomized. No special or separate head is needed to create erase bands, as the randomized magnetic moments are a product of the edge effects of the read/write head.
During reading, the lack of a signal from the erase band will allow the read/write head to move off-center with respect to the center of the track to a certain degree without the read operation being corrupted by the data of adjacent track portions. The degree to which a read/write head can move off-center of the track and still read data accurately is known as off-track capability. The bit error rate (BER) of data read by the read/write head while off-track is known as off-track performance.
Erase bands can therefore be beneficial for off-track performance. However, it is not beneficial to make the width of erase bands arbitrarily large. Given the finite storage area available on magnetic media such as hard disks, increasing the erase band width (EBW) will decrease track density (the number of tracks per unit length) and the overall data storage capability of the disk. High track density can be maintained by narrowing the track widths, but as track width decreases, the output signal during readback will also be reduced. As the output signal is reduced, the signal-to-noise ratio (SNR) will decrease as noise from other sources such as the channel electronics and the head becomes larger with respect to the output signal, eventually reaching a point sufficient to corrupt the signal being read. Thus, the tradeoffs between track density, track width, off-track performance, and BER establish a practical upper limit to EBW.
Another factor affecting EBW is the linear density (the number of flux reversals per unit length) of the recorded disk. Data storage capability can be increased by writing more information (more flux reversals) in a given length of track, thereby increasing the linear density. As indicated in FIG. 5, as information is written with increased linear density, the EBW of the simultaneously created erase bands increases. Thus, maximizing data storage capability can be a complex process involving tradeoffs between the competing parameters of linear density, track density, EBW, track width, off-track performance, and BER. Measurement of EBW is therefore an important task in the research and development of magnetic media products.
Several methods are currently used in EBW measurement. One method utilizes magneto-force microscopy (MFM), wherein the magnetic force between a magnetized tip and the magnetic media is sensed and an "image" is formed to determine the boundaries and widths of erase bands and tracks. A disadvantage of MFM methods is that it takes a long time to obtain one measurement, making repeated measurements impractical.
A faster method is disclosed in the article "Effects of the Increase of Side Erase Band Width on Off-Track Capability of High Frequency Magnetic Recording" by Huang, Yeo, and Tran in IEEE Transactions on Magnetics, Vol. 32, No. 5, September 1996. In that method, the magnetic media is DC-erased, two reference tracks are written at the same linear density, and a desired track is written over the two background reference tracks, also at the same linear density. A track scan is then performed (moving a read head radially across a track) in which a narrow-band overwrite filter is used to measure the narrow band read back signal. The resultant signal profile as a function of the read head position is then computer-extrapolated, and after some calculations the EBW can be approximated.
Another method is disclosed in the article "Effects of Current and Frequency on Write, Read, and Erase Widths for Thin-Film Inductive and Magnetoresistive Heads" by Lin, Christner, Mitchell, Gau, and George in IEEE Transactions on Magnetics, Vol. 25, No. 1, January 1989. In that method, the magnetic media is DC-erased, two reference tracks are written at the same linear density, and a desired track is written over the two background reference tracks at a different linear density. A track scan is then performed with two spectrum analyzers independently tuned to the frequency of the reference and desired tracks, and the resultant read head output voltage profile of the three tracks is used to calculate an approximate EBW.